HOBSON and CHESS; TROPHIC RELATIONSHIPS AMONG FISHES 



site, a slow outflow from the lagoon was evident in 

 East Channel itself. Even though strong currents 

 at this site were limited essentially to flooding 

 spring tides, their impact was clearly visible on 

 the substrate at all times. Most notable, the sand, 

 which swirled about in the stronger currents, was 

 piled high in the lee of the patch reefs. 



METHODS 



Plankton 



The methods used to collect plankton differed 

 between the two primary sites owing to the con- 

 trast in prevailing current velocities. Never- 

 theless, all collections employed the same 

 0.333-mm mesh net and produced comparable as- 

 sessments of the plankton at the two places, par- 

 ticularly between day and night. 



Collecting Where Currents Were Weak 



When sampling at the Walt Island site, we 

 pushed the net through the water around one 

 patch reef (Figure 2), a circuit that always took 5 

 min. The procedure was similar to that used at 

 Santa Catalina Island, Calif. (Hobson and Chess 

 1976). When swimming with the net by day, we 

 could watch organisms in its path, and this gave us 

 insight into which of them might be evading the 

 net. Mysids, for example, could do so, and often 

 did. But these organisms reacted to us less than 

 expected, perhaps because the meter net's opening 



was large, and its approach was slow and quiet. 

 Certainly our collections would have sampled 

 these large mobile forms less effectively if the net 

 had been preceded by the harness and tow line 

 used when operating from a boat. 



Three series of collections were made during 

 midday (between 1000 and 1400 h), and three 

 series were made at night ( 1 h after last evening 

 light, at midnight, and 1 h before first morning 

 light). We spaced the noctunal collections over the 

 night because earlier work had suggested that 

 certain organisms are in the water column only 

 briefly during specific periods of the night, a 

 phenomenon we did not find among the diurnal 

 plankters (Hobson and Chess 1976). Of the three 

 collections in each series, one was made within 1 m 

 of the bottom, one midway between bottom and 

 surface, and one with the net breaking the surface. 

 At night, ambient light in this clear water over 

 white sand permitted us to collect without diving 

 lights. Our stay at Enewetak spanned the period 

 from full to new moon, so that we sampled both 

 spring and neap tides, but generally there was no 

 moonlight during the collections owing to cloud 

 cover or time of night. 



Net speed was 28 cm/s, as calculated from read- 

 ings of a current meter calibrated by the speed at 

 which the smallest fragments of algae visible to us 

 drifted along a measured course. We decided it was 

 necessary to determine net speed only once at this 

 station, because all collections were made by the 

 same two swimmers who each time exerted about 

 the same effort, and covered the same distance. 



FIGURE 2.— Collecting plankton at 

 Walt Island, Enewetak Atoll, site of 

 weak currents. The square frame per- 

 mitted more accurate assessments at 

 the surface and close to the sea floor. 



135 



